On Tue, Jan 12, 2021 at 10:57:07AM -0800, Andy Lutomirski wrote: > On Tue, Jan 12, 2021 at 10:24 AM Luck, Tony <tony.luck@xxxxxxxxx> wrote: > > > > On Tue, Jan 12, 2021 at 09:21:21AM -0800, Andy Lutomirski wrote: > > > Well, we need to do *something* when the first __get_user() trips the > > > #MC. It would be nice if we could actually fix up the page tables > > > inside the #MC handler, but, if we're in a pagefault_disable() context > > > we might have locks held. Heck, we could have the pagetable lock > > > held, be inside NMI, etc. Skipping the task_work_add() might actually > > > make sense if we get a second one. > > > > > > We won't actually infinite loop in pagefault_disable() context -- if > > > we would, then we would also infinite loop just from a regular page > > > fault, too. > > > > Fixing the page tables inside the #MC handler to unmap the poison > > page would indeed be a good solution. But, as you point out, not possible > > because of locks. > > > > Could we take a more drastic approach? We know that this case the kernel > > is accessing a user address for the current process. Could the machine > > check handler just re-write %cr3 to point to a kernel-only page table[1]. > > I.e. unmap the entire current user process. > > That seems scary, especially if we're in the middle of a context > switch when this happens. We *could* make it work, but I'm not at all > convinced it's wise. Scary? It's terrifying! But we know that the fault happend in a get_user() or copy_from_user() call (i.e. an RIP with an extable recovery address). Does context switch access user memory? -Tony
